Method and device for cool-drying

ABSTRACT

The present invention concerns a method for cool-drying gas containing water vapor, whereby this gas is guided through the secondary part of a heat exchanger ( 1 ) whose primary part is the evaporator ( 2 ) of a cooling circuit ( 3 ) which also contains a compressor ( 5 ) which is driven by an electric motor ( 4 ), a condenser ( 6 ), an expansion means ( 7 ) between the outlet of the condenser ( 6 ) and the inlet of the evaporator ( 2 ), and whereby the above-mentioned cooling circuit ( 3 ) is thus controlled as a function of the load that the cooling capacity is adjusted without any ice being formed in the evaporator ( 2 ), characterized in that the cooling circuit is controlled by adjusting the rotational speed of the motor ( 4 ).

BACKGROUND OF THE INVENTION

The present invention concerns a method for cool-drying gas containingwater vapor, whereby this gas is guided through the secondary part of aheat exchanger whose primary part is the evaporator of a cooling circuitwhich also contains a compressor which is driven by an electric motor, acondenser, an expansion means between the outlet of the condenser andthe inlet of the evaporator, and whereby the above-mentioned coolingcircuit is thus controlled as a function of the load that the coolingcapacity is adjusted without any ice being formed in the evaporator.

Such methods are used among others for drying compressed air.

Compressed air which is supplied by a compressor is in most casessaturated with water vapor or, in other words, has a relative humidityof 100%. This implies that there is condensation at the slightestdecrease of temperature. The water of condensation causes corrosion inthe pipes and tools, and the equipment will wear out prematurely.

That is why the compressed air is dried, which may be done in theabove-mentioned manner, by means of cool-drying. Also other air thancompressed air or other gases may be dried in this manner.

Cool-drying is based on the principle that, by lowering the temperature,moisture from the air or the gas condenses, after which the water ofcondensation is separated in the liquid separator and after which theair or the gas is heated again, as a result of which this air or thisgas is no longer saturated. The heat is discharged by the coolingcircuit in the evaporator.

The same applies to other gases than air, and each time air is referredto hereafter, the same also applies to other gases than air.

In practice, there is an ISO-standard which determines the possible dewpoint and the corresponding lowest air temperature for reference values.

SUMMARY OF THE INVENTION

In order to prevent the lowest air temperature from dropping below 0° C.and thus the evaporator from freezing up, a necessary condition is thatthe temperature of the evaporator is higher than 0° C.

According to known methods, to this end, the temperature is measured onthe inlet of the evaporator, or, since there is a definite connectionbetween the temperature of the evaporator and the pressure of theevaporator for a specific cooling liquid in the cooling circuit, thepressure is measured before or after the evaporator.

The cooling circuit is then controlled such that the temperature of theevaporator or the pressure of the evaporator has the required value, andfor example the pressure of the evaporator coincides with a temperaturewhich is situated a few degrees below the required lowest airtemperature or LAT, but not below 0° C.

According to these known methods for cool-drying, the motor of thecompressor of the cooling circuit which is driven at a constantfrequency is switched on and off as a function of the temperature of theevaporator. If this pressure of the evaporator decreases too much, saidmotor is stopped. If the pressure of the evaporator subsequentlyincreases too much as the expansion valve is still open, the motor isstarted again.

Such a regulation makes it possible for the compressor to be switchedoff when the load drops beneath the cooling capacity, as a result ofwhich the energy consumption will decrease. The surplus of coolingcapacity is stored in a thermal mass. However, this regulation is verydisadvantageous as the compressor is continuously switched on and off incase of a small load, while also the pressure of the evaporator and thedew points fluctuate strongly. Moreover, the cool-dryer must be builtrelatively large.

Another known method consists in measuring the lowest air temperature(LAT) on the outlet of the secondary part of the heat exchanger and toswitch off the motor of the compressor of the cooling circuit when thetemperature threatens to drop below 0° C. This method, whereby the motoris thus also switched on and off, offers the same disadvantages as thepreceding one.

Another possibility for regulating the pressure of the evaporator wouldconsist in selecting an evaporator which is large enough and in carryingback hot gases on the outlet of the compressor to the inlet of thecompressor by means of a bypass.

This regulation method is disadvantageous in that, since the compressormotor is continuously working, also when there is no load or when theload is low, the energy consumption is equal to the energy consumptionat a nominal load, as the high and low pressure in the cooling circuitare continuously kept at a constant level.

The object of the invention is a method for cool-drying which does nothave the above-mentioned and other disadvantages and which makes itpossible to save energy in a simple manner, without any pressurevariations in the cooling circuit and without much wear of thecompressor and its motor.

In accordance with the invention, this object is accomplished in thatthe cooling circuit is controlled by adjusting the rotational speed ofthe motor.

Instead of switching the motor on or off, its speed is adjusted. Byincreasing the rotational speed of the motor, more mass flow of coolingliquid can be pumped round, and thus can be obtained a higher coolingoutput.

The temperature of the evaporator can be measured and theabove-mentioned cooling circuit can be controlled as a function of themeasured temperature of the evaporator.

According to another embodiment, the pressure of the evaporator can bemeasured and the above-mentioned cooling circuit can be controlled as afunction of the measured pressure of the evaporator.

According to yet another embodiment, the lowest gas temperature (LAT)can be measured and the above-mentioned cooling circuit is controlled asa function of this lowest gas temperature (LAT).

According to yet another embodiment, the dew point temperature of thegas can be measured and the above-mentioned cooling circuit iscontrolled as a function of this dew point.

Preferably, the rotational speed of the motor is adjusted by modifyingthe frequency of the supply current.

According to a special embodiment of the invention, the ambienttemperature is measured and the rotational speed of the motor isadjusted as a function of the measured ambient temperature.

At high ambient temperatures, whereby the air or the gas is alsorelatively warm and may contain more moisture than when it is cold, itis not necessary to cool it to 3° C. in the heat exchanger in order toobtain dry air. Thus, the energy consumption of the above-mentionedcool-dryers is too high, and they require relatively large and expensivecomponents in order to supply the cooling output. By taking into accountsaid ambient temperature, the required cooling output may be kept lower,such that the cool-dryer can be made less large.

Preferably, the rotational speed of the motor of the compressor isadjusted such that the lower air or gas temperature on the outlet of theevaporator is 20° C. lower than the measured ambient temperature,without dropping below 3° C., however.

It is assumed that, when the outgoing air or the outgoing gas has arelative humidity of 50%, the danger of corrosion in pipes and equipmentis excluded, and the above-mentioned control device guarantees that saidrelative humidity will not be higher than 50%.

The invention also concerns a device for cool-drying or a cool-dryerwhich is particularly suitable for applying the above-mentioned method.

The invention in particular concerns a device for cool-drying,containing a heat exchanger whose primary part is the evaporator of acooling circuit which also contains a compressor which is driven by anelectric motor, a condenser, an expansion means between the outlet ofthe condenser and the inlet of the evaporator, a control device tocontrol the above-mentioned motor and measuring means coupled thereto,whereas the secondary part of the heat exchanger is part of a pipe forthe gas and a liquid separator is erected on the outlet of said heatexchanger, in said pipe, whereby the device contains means to adjust therotational speed of the motor while the control device controls thesemeans as a function of the value measured by the measuring means.

The measuring means may be provided on the cooling circuit and they maybe means to measure the temperature of the evaporator or the pressure ofthe evaporator.

However, the measuring means may also be provided on the pipe for thegas, in the secondary part of the heat exchanger or downstream to it,and they may be means to measure the lowest gas temperature (LAT) ormeans to measure the dew point.

Preferably, the means for regulating the rotational speed of the motorconsist of a frequency converter.

According to a special embodiment of the invention, the cool-dryercontains means for measuring the ambient temperature which are alsocoupled to the control device, and this control device is such that itadjusts the speed of the motor both as a function of the value measuredby the measuring means and as a function of the temperature measured bythe means for measuring the ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better explain the characteristics of the invention, thefollowing preferred embodiments of a cool-dryer according to theinvention are described as an example only without being limitative inany way, with reference to the accompanying drawings, in which:

FIG. 1 represents a block diagram of a device for cool-drying accordingto the invention;

FIG. 2 represents a block diagram analogous to that of FIG. 1, but inrelation to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The device for cool-drying which is schematically represented in FIG. 1mainly contains a heat exchanger 1 whose primary part forms theevaporator 2 of a cooling circuit 3 in which are also successivelyerected a compressor 5 driven by electric motor 4, a condenser 6 and anexpansion valve 7.

This cooling circuit is filled with cooling fluid, for example freon404a, whose direction of flow is represented by the arrow 8.

The secondary part 1A of the heat exchanger 1 is part of the pipe 9 forhumid air to be dried, whose direction of flow is represented by thearrow 10.

After the heat exchanger 1, i.e. on its outlet, a liquid separator 11 iserected in the pipe 9.

Before it reaches the heat exchanger 1, this pipe 9 may possibly extendthrough a pre-cooler or recuperation heat exchanger 12 with one part andsubsequently, after the liquid separator 11, extend again through therecuperation heat exchanger 12, counterflow to the above-mentioned part.

The heat exchanger 1 is a liquid/air heat exchanger and, from aconstructional point of view, may form a whole with the possiblerecuperation heat exchanger 12 which is an air/air heat exchanger.

The expansion valve 7 is a thermostatic valve whose thermostatic elementis coupled to a bulb 14 provided on the outlet of the evaporator 2 onthe cooling circuit 3 by means of a copper guide 13 and which is filledwith the same cooling liquid.

According to a variant which is not represented in the figure, thisexpansion valve is an electronic valve, however, which is coupled to atemperature gauge erected on the far end of the evaporator 2 or afterit.

In small cool-dryers, the expansion valve 7 may be replaced by acapillary tube.

The compressor 5 is a volumetric compressor which supplies an almostinvariable volume flow at an invariable rotational speed, for example aspiral compressor, whereas the motor 4 is an electric motor whoserotational speed can be adjusted by changing the frequency.

Also, this motor 4 is coupled to a frequency converter 15 which iscontrolled by a control device consisting of a built-in PID controller16.

The frequency converter 15 may for example adjust the frequency between0 and 400 Hz and forms means to adjust the rotational speed of the motor4.

According to a first embodiment, the PID controller 16 is connected to atemperature measuring means 18 via a pipe 17 to measure the pressure ofthe evaporator, for example a pressure transmitter with a pressure rangefrom −1 to 12 bar which transforms the pressure in an electric signal,in particular a current, which is erected on the inlet or the outlet ofthe evaporator 2, as is represented in the figure by means of a dashedline.

According to a second embodiment, the PID controller 16 is connected tothe temperature measuring means 20 via a connecting means 19 to measurethe temperature of the evaporator, for example a thermocouple in thecooling circuit 3, on the inlet of the evaporator 2 and thus betweenthis evaporator 2 and the expansion valve.

Indeed, for a given cooling fluid, there is a definite connectionbetween the temperature of the evaporator and the pressure of theevaporator. The higher the temperature, the higher the pressure.Strictly speaking, this connection is not linear, but in the field ofoperation, i.e. between 0° C. and 25° C., the deviation from a linear isto be practically neglected.

In both embodiments, the PID controller 16 is connected to an ambienttemperature means 22 via a pipe 21 to measure the ambient temperatureand which transforms this temperature into an electric signal, inparticular a current.

The working of the cool-dryer is as follows:

The air to be dried is carried through the pipe 9 and thus through theheat exchanger 1, counterflow to the cooling fluid in the evaporator 2of the cooling circuit 3.

In this heat exchanger 1, the damp air is cooled, as a result of whichcondensation is formed which is separated in the liquid separator 11.

The cold air, which contains less moisture after this liquid separator11 but yet has a relative humidity of 100%, is heated in therecuperation heat exchanger 12, as a result of which the relativehumidity decreases to about 50%, while the fresh air to be dried isalready partly cooled in this heat exchanger 12 before being supplied tothe heat exchanger 1.

The air on the outlet of the recuperation heat exchanger 12 is thusdrier than on the inlet of the heat exchanger 1.

In order to prevent the evaporator 2 from freezing, the air in the heatexchanger 1 is not cooled below 3° C., which is the LAT for low ambienttemperatures.

With higher ambient temperatures, the LAT may be higher and may becooled to an LAT which is 20° C. lower than the ambient temperature, andin any case not below 3° C.

If the LAT is too high, this means that there is not enough cooling andthus not enough condensation of moisture to sufficiently dry the air.

Said LAT is situated 2 to 3° C. above the actual temperature of theevaporator which is measured by the measuring means 20.

The above-mentioned LAT conditions are met by adjusting the rotationalspeed of the motor 4 as a function of the temperature of the evaporatormeasured by the measuring means 20 by means of the PID controller 16 andthe frequency converter 15 controlled by it in the one embodiment, orthe pressure of the evaporator measured by the measuring means 18 in theother embodiment.

The cooling output is equal to the mass flow of cooling liquidcirculating in the cooling circuit 3, multiplied by the enthalpydifference of the air before and after the heat exchanger 1. Byincreasing the rotational speed of the motor 4, the compressor 5 canpump round more mass flow, and thus can be supplied a larger output withthe same enthalpy difference. The mass flow is the volume flow of thecompressor 5, multiplied by the density of the cooling liquid in thesuction condition, which itself depends on the temperature of theevaporator and the overheating.

The PID controller 16 adjusts the measured temperature or pressure byadjusting the rotational speed, so that this temperature is a fewdegrees lower than the above-mentioned LAT, but yet higher than 0° C.,the pressure of the evaporator is reached respectively, which coincideswith a temperature which is a few degrees lower than the LAT and whichis for example equal to 1° C., whereby for freon R404a, the pressure ofthe evaporator is effectively about 5.2 bar.

In this manner, the cooling output is adjusted to the load.

As the means 22 also measure the ambient temperature, the PID controller16 coupled to it can take this temperature into account.

By means of the PID controller 16 and the frequency converter 15controlled by it, the rotational speed of the motor 4 is than adjustedsuch that, as long as the ambient temperature is low, in particularbelow 23° C., the above-mentioned condition is met and thus the LAT onthe outlet of the secondary part 1A of the heat exchanger 1 is about 3°C., but at a higher ambient temperature, this LAT is 20° C. lower thanthe ambient temperature measured by the means 21.

The temperature of the evaporator has a set point which is a few degreeslower than the required LAT. The temperature which is obtained bysubtracting some 22° C. from the ambient temperature, can be calibratedas the set point of the PID controller 16.

A minimum and a maximum set point may possibly be set in the PIDcontroller 16, whereby the minimum is 1° C. When calibrating the PIDcontroller 16, this set point can be adjusted for example via a controlpanel or via an analogous inlet.

The frequency is adjusted between for example 30 and 75 Hz.

The maximum load of the cool-drying device is relatively small, since,at higher ambient temperatures, the LAT can be higher than 3° C., as aresult of which the cooling output decreases and the components may thusbe less expensive and cooling fluid is saved on.

In the condenser 6, the cooling fluid which has been heated in thecompressor 5 as a result of the compression, is cooled until it has aliquid state, whereby use can be made of a fan or of cooling water todischarge the heat to the environment.

When the pressure in the condenser 6 is too high, the motor 4 isautomatically switched off.

After the condenser 6, the liquid cooling fluid may possibly becollected in a receptacle and/or it may be further cooled by an extraheat exchanger.

Thanks to the expansion valve 7, the liquid cooling fluid is expanded toa constant evaporator pressure, which of course results in a temperaturedecrease.

The expansion valve 7 only controls the overheating in the evaporator 2and makes sure that the evaporator 2 is always optimally used, but itcannot be used to control the pressure of the temperature of theevaporator.

By applying a thermostatic expansion valve 7, there will always beoverheating after the evaporator 2, so that there is no danger ofcooling liquid entering the compressor 5, so that there is no need for aliquid separator in the cooling circuit 3 and so that the amount ofcooling fluid is restricted.

This overheating is measured by subtracting the temperature measured bythe bulb 14 from the temperature of the evaporator, either before theevaporator 2 (internal equalization) or after the evaporator (externalequalization). This difference is compared to a predetermined value bythe expansion valve 7 and, in case of a deviation, the expansion valve 7will correct it by opening or by closing.

The degree of overheating has an influence on the LAT, but we may assumethat this overheating is kept at a practically constant level by theexpansion valve.

If necessary, this influence of the overheating can be taken intoaccount by for example a sort of master/slave control circuit. The slavecontrol circuit is the above-described control with the PID controller16, whereas the master control circuit could adjust the set point of thepressure or temperature of the evaporator as a function of the actualLAT, and thus could for example lower the set point if the LAT remainstoo high as the overheating after the evaporator 2 is too high.

Although the pressure or temperature of the evaporator is adjusted bymodifying the rotational speed, it may be possible to entirely switchoff the motor 4 in case the load is zero, for example by placing athermostatic sensor in the heat exchanger 1 which, should thetemperature in the evaporator 2 drop to zero degrees, switches off themotor 4 and starts it again as soon as the temperature has risen to 3°C.

The embodiment of the invention represented in FIG. 2 mainly differsfrom the above-described embodiments in that the measuring means 18 formeasuring the pressure of the evaporator and/or the measuring means 20for measuring the temperature of the evaporator provided on the coolingcircuit 3 have been replaced by temperature measuring means 23 formeasuring the lowest air temperature (LAT).

These measuring means 23 have been provided on the pipe 9, either in thesecondary part 1A of the heat exchanger 1, for example on the surface ofthe evaporator 2, or as represented in FIG. 2, downstream the heatexchanger 1, for example between this heat exchanger 1 and the liquidseparator 11.

The PID controller 16 is then connected to these measuring means 23 andto the means 22 for measuring the ambient temperature by means of a pipe21.

In this embodiment, the PID controller 16 controls the frequencyconverter 15 and thus the rotational speed of the motor 4 as a functionof the measured lowest air temperature LAT.

Measuring the LAT offers a major advantage in that the temperature ofthe cooling fluid may be lower than 0° C. without the evaporator therebyfreezing up, i.e. before ice is formed on the air side of theevaporator, since this phenomenon is determined by the LAT.

Since with low evaporator temperatures, for example −5° C., on the sideof the cooling fluid, and major temperature differences, for example of8° C. (between +3° C. and −5° C.), there may be evaporation without anydanger of freezing, the heat exchanger 1 can be made very compact.

If the measured lowest air temperature LAT rises or drops, the PIDcontroller 16 will order the speed of the motor 4 to increase, decreaserespectively, such that, as long as the ambient temperature measured bythe temperature gauge 23 is low, in particular lower than 23° C., thismeasured LAT temperature will not drop below some 3° C., so as to makesure that the evaporator 2 will not freeze up.

Thanks to this control, the cooling is thus adjusted according to theload, whereby the evaporator temperatures on the side of the coolingfluid may drop below zero without the evaporator 2 freezing up on theair side, however. As a result, not only the energy consumption of themotor 4 is restricted to a minimum, but the heat exchanger 1 can be maderelatively compact, which also implies savings on the cost of thedevice.

In this embodiment also, the overheating in the evaporator 2 iscontrolled by the expansion valve 7, as a result of which the coolingfluid expands.

Although the lowest air temperature is adjusted by modifying therotational speed of the motor 4, it may be possible in this embodimentas well to entirely switch off the motor 4 in case of a zero load.

According to a variant of the preceding embodiment which is notrepresented in the figures, the measuring means 23 for measuring thelowest air temperature are replaced by measuring means for measuring thedew point of said air. Such measuring means or dew point gauges areavailable on the market and hence are not further described here.

Instead of the LAT, the dew point of the air is thus measured on thesame place. The working is analogous to the above-described working,whereby the speed of the motor 4 is thus adjusted such that the coolingin the heat exchanger 1 is optimal, but the evaporator 2 is preventedfrom freezing up.

The invention is by no means limited to the above-described embodimentsrepresented in the accompanying drawing; on the contrary, such a methodand device for cool-drying can be made in all sorts of variants whilestill remaining within the scope of the invention.

In particular, instead of a PID controller 16, the control device maycontain another controller, for example a PI or P controller. Althoughthe ambient temperature is preferably also taken into account, amongothers to restrict the output of the device, it is possible, accordingto a simpler embodiment, to adjust the rotational speed of the motor 4merely as a function of the evaporator temperature, the evaporatorpressure, the lowest gas temperature or the dew point of the gas.

Instead of damp air, other gases than air containing water vapor can bedried in the same manner and with the same device. The LAT is then thelowest gas temperature.

I claim:
 1. A method for cool-drying gas containing a water vapor, saidgas being guided through a heat exchanger having primary and secondaryparts, said primary part of said heat exchanger being an evaporatorbelonging to a cooling circuit having a cooling medium disposed thereinthat includes a compressor driven by a motor, a condenser, and anexpansion device positioned between the outlet of the condenser and theinlet of the evaporator, said method comprising the steps of: guidingthe gas through the secondary part of the heat exchanger, said secondarypart of the heat exchanger in communication with said primary partthereof; cooling the gas as said gas passes through said heat exchanger;controlling the temperature of the cooling circuit such that ice is notformed in the evaporator, said temperature being controlled by adjustingthe rotational speed of the motor as a function of the measuredtemperature of the gas or the evaporator; and separating condensed waterfrom said gas in a liquid separator as said gas exits said heatexchanger.
 2. The method according to claim 1 further comprising thestep of measuring the temperature of said evaporator such that saidcooling circuit is controlled as a function of the measured evaporatortemperature.
 3. The method according to claim 2 wherein the rotationalspeed of the motor is adjusted such that the evaporator temperature isset 2 to 3° C. below the lowest gas temperature (LAT).
 4. The methodaccording to claim 2 wherein the temperature of the evaporator ismeasured at the inlet of the evaporator in said cooling circuit suchthat an evaporation temperature of the cooling medium within the coolingcircuit is measured.
 5. The method according to claim 1 furthercomprising the step of measuring the lowest gas temperature (LAT) suchthat the temperature of the cooling circuit is controlled as a functionof the lowest gas temperature (LAT).
 6. The method according to claim 5wherein the lowest gas temperature (LAT) is measured at the outlet ofthe secondary part of the heat exchanger.
 7. The method according toclaim 1 further comprising the step of measuring the dew point of thegas such that the temperature of the cooling circuit is controlled as afunction of the dew point of the gas.
 8. The method according to claim 1wherein the cooling circuit is controlled such that the temperaturealong a side of the evaporator in communication with the cooling mediumdrops below zero without said evaporator freezing along another sidethereof in communication with said gas.
 9. The method according to claim1 wherein the rotational speed of the motor is adjusted by modifying thefrequency of a supply current.
 10. The method according to claim 1further comprising the step of measuring the ambient temperature,wherein the rotational speed of the motor is adjusted on the basis ofthe measured ambient temperature.
 11. The method according to claim 10further comprising the step of adjusting the rotational speed of themotor of the compressor such that the lowest adjustable temperature(LAT) on the outlet evaporator is set between 3° C. and 20° C. below themeasured ambient temperature.
 12. The method according to claim 1further comprising the step of expanding the cooling medium in anexpansion valve before the cooling medium enters the evaporator, whereinoverheating of the cooling medium is measured after said cooling mediumpasses through the evaporator and is compared to a predetermined valuesuch that in the event of a deviation between the predetermined valueand the measured value, said expansion valve either opens or closesdepending upon the degree of deviation.
 13. The method according toclaim 1 further comprising the step of drying said gas in a recuperationheat exchanger after said gas passes through said heat exchanger andsaid liquid separator.
 14. A cool-drying device configured to cool-dry agas containing a water vapor, said device comprising: a heat exchangerhaving primary and secondary parts; said primary part being anevaporator belonging to a cooling circuit having a cooling mediumdisposed therein that includes a compressor driven by a motor, acondenser, an expansion device positioned between the outlet of thecondenser and the inlet of the evaporator, and a temperature measuringdevice; and said secondary part including a pipe in communication withsaid primary part of said heat exchanger and arranged for said gas toflow therethrough, and a liquid separator positioned along said pipeafter said pipe exits said evaporator; an adjustment device arranged toadjust the rotational speed of said motor; and a control device arrangedto control said adjustment device as a function of the temperature ofsaid gas or the evaporator as measured by said temperature measuringdevice.
 15. The device according to claim 14 wherein the temperaturemeasuring device is positioned in said cooling circuit and is arrangedto measure the temperature in said evaporator.
 16. The device accordingto claim 14 wherein the temperature measuring device is positioned alongsaid pipe in cooperation with the gas in or downstream said secondarypart, said temperature measuring device measuring the lowest gastemperature (LAT).
 17. The device according to claim 14 wherein saidtemperature measuring device is provided along the pipe in cooperationwith the gas in or downstream said secondary part of the heat exchanger,said temperature measuring device measuring the dew point of said gas.18. The device according to claim 14 wherein the adjustment deviceincludes a frequency converter.
 19. The device according to claim 14further comprising an ambient temperature measuring device arranged tomeasure the ambient temperature near said cool-drying device and coupledto the control device, said control device adjusting the rotationalspeed of the motor as a function of the values measured by saidtemperature measuring device and the ambient temperature measuringdevice.
 20. The device according to claim 14 wherein the control deviceis a PID control, a PI control or a P control.